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How to connect my transformer to mains power properly

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mkelly09:
Hello everyone,

I've just started undertaking a DIY bench power supply design based on this one here:
http://electronics-diy.com/electronic_schematic.php?id=787

Schematic here: http://electronics-diy.com/schematics/787/Bench_Supply_Schematic.pdf

Before i get into this i would like to mention that I have read up on the threads here regarding mains safety and will be very careful when doing this project.  I realize that it can be dangerous, but at the same time, I am doing this to learn as much as i can.  I feel that i will learn more by actually using a transformer vs. using a wall wart

So far, the features that I know I want( a bit different than the project linked):

-3 outputs: +15V, -15V (for op-amps) and one variable output 0-15(or 20, 25 but really no need for this much)
-1A output (flexible i guess)
-LCD displaying the voltage and current for my variable output
-pot for voltage adjustment, on/off switch connected to shut off mains, on/off switch for the +15 and -15 outputs(one switch for both), and one last switch for the variable output
-probably some LEDS to indicate power on for my outputs(or i could get those lighted switches)
-2A fuse connected on mains before switch
-some form of current limiting circuit

Here's the transformer i've bought: http://parts.digikey.com/ca/1/621291-xfrmr-pwr-115-230v-17vac-2-8a-lp-34-1400.html
its datasheet is located here: http://www.signaltransformer.com/sites/all/pdf/LP.pdf

My question is how do i connect this transformer correctly, as there are four pins on each side.  Is there an easy way to tell which side of the transformer is the primary side(one is marked signal, the other something else- i don't have it in front of me ATM)

From digi-key:
Series Output Voltage @ Current 34 VAC @ 1.4A
Parallel Output Voltage @ Current 17 VAC @ 2.8A

I figure i'd like the series output voltage. 

Also, while i'm here, whats the best option to do with the green(earth) wire?  I plan on mounting this in a wood case, should i just snip it back and heat shrink it?

bilko:
The transformer you selected won't allow you to load up the supplies. The maximum current is 1.4A, this is for all the outputs combined. If this is OK for you, I would fuse the secondary with a 2A fuse to protect the transformer.
The ground wire from the mains should be connected to the frame of the transformer. You can use one of the mounting bolts with a tag soldered onto the ground wire.

Psi:
In the picture it looks to have numbering on the label to show you which pin is what.
From the datasheet pins 1,2,3,4 are the primary.
It has two 115V coils which can be connected in parallel for 115v operation or in series if you have 230v

For 230V you would connect pins 2 and 3 together and feed 230V between pins 1 and 4
For 115V you would connect pins 1 and 3 together and this would be one 115v input. The other would be pins 2 and 4 (connected together)


If there is no label on your one then there is a trick you can do to work it out, but it requires wiring up 110/230v so beware of safety issues.
There is probably a safer way to do it, but this is how i do it.

Pick one side and connect it to mains as above (for 110 or 230V) but put a 110/220v light bulb in series with it. That way, if it's a dead short you don't blow up anything. Instead the lightbulb will just light up and tell you you have it connected wrong.
Make sure the other side of the transformer is left unconnected, if it turns out to be the 230v side it will have 230v on it!

This works because when a transformer is connected correctly and there is no load it should draw no current. No current means the lightbulb will stay off.

Of course to do this you need to have a 110/230v lightbulb holder and you have to be very careful to not touch anything when the power is on. Only work on it when the power is off and its unplugged from the wall.
Safety first!

Uncle Vernon:

--- Quote from: mkelly09 on May 19, 2011, 01:25:35 pm ---Also, while i'm here, whats the best option to do with the green(earth) wire?  I plan on mounting this in a wood case, should i just snip it back and heat shrink it?

--- End quote ---

Not being rude, but this question screams to me that your not quite ready to be building mains powered projects unassisted just yet.

Any exposed metallic parts of your project should be securely earthed as an absolute minimum.

I'd suggest you look at some more detailed projects for some clues about securing mains cords, chassis and frame earthing etc.

The transformer you've selected is a PCB mounting type, how did you intend on mounting it?

I don't want to dampen your enthusiasm, but maybe the wall-wart approach would be better for a first project.

Mechatrommer:
safer way without messing with mains, use a dmm, even the cheapo will happily do. check the ohm at each pin. the connection with almost 0 ohm is secondary. primary will have more resistance like 10-200 ohm.

you can get away without earth wire. earth wire is meant to tunnel any stray current (from life) back to earth safely. so if your design has no leak, your system will run without fault. but in case life wire leaks/connected to gnd part of the circuit or equipment metalic housing, with earth wire connected, you will not risk touching the housing and the stray current running through you back to earth. but certainly earth wire is not meant to be connected to wood, it will not conduct current anywhere.

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